Dimorphic ErAgSn and TmAgSn – high‐pressure and high‐temperature driven phase transitions

Abstract

The stannides ErAgSn and TmAgSn have been investigated under high-temperature (HT) and high-pressure (HP) conditions in order to investigate their structural chemistry. ErAgSn and TmAgSn are dimorphic: normal-pressure (NP) ErAgSn and HT-TmAgSn crystallize into the NdPtSb type structure, P63mc, a = 466.3(1), c = 729.0(2) pm for NP-ErAgSn and a = 465.4(1), c = 726.6(2) pm for HT-TmAgSn. NP-ErAgSn was obtained via arc-melting of the elements and subsequent annealing at 970 K, while HT-TmAgSn crystallized directly from the melt by rapidly quenching the arc-melted sample. HT-TmAgSn transforms to the ZrNiAl type low-temperature modification upon annealing at 970 K. The high-pressure (HP) modification of ErAgSn was synthesized under multianvil high-pressure (11.5 GPa) high-temperature (1420 K) conditions from NP-ErAgSn: ZrNiAl type, P‾62m, a = 728.7(2), c = 445.6(1) pm. The silver and tin atoms in NP-ErAgSn and HT-TmAgSn build up two-dimensional, puckered [Ag₃Sn₃] networks (277 pm intralayer Ag–Sn distance in NP-ErAgSn) that are charge-balanced and separated by the erbium and thulium atoms. The fourth neighbor in the adjacent layer has a longer Ag–Sn distance of 298 pm. The [AgSn] network in HP-ErAgSn is three-dimensional. Each silver atom has four tin neighbors (281–285 pm Ag–Sn). The [AgSn] network leaves distorted hexagonal channels, which are filled with the erbium atoms. The crystal chemistry of the three phases is discussed.